Disk drives are popular and cost effective data storage systems for a computer or other data processing device. As shown in FIG. 1, a disk drive 10 comprises a magnetic recording medium, in the form of a disk or platter 12 having a hub 13 and a magnetic read/write transducer 14, commonly referred to as a read/write head. The read/write head 14 is attached to, or formed integrally with, a suspension arm 15 suspended over the platter 12 and affixed to a rotary actuator arm 16. A structural arm 18 is fixed to a platform 20 and pivotably connected to the actuator arm 16 at a pivot joint 22. A voice coil motor 24 drives the actuator arm 16 to position the head 14 over a selected location on the disk 12 for reading data from or writing data to the disk 12.
As the disk 12 is rotated by a spindle motor (not shown) at an operating speed, air flow generated by the rotating disk, in conjunction with the physical features of the suspension arm 15, produces lift for displacing the read/write head 14 above the platter 12, allowing the head to glide on a cushion of air slightly above an upper surface of the platter 12. The flying height of the read/write head over the disk surface is typically less than a micron. A preamplifier 30, electrically connected to the head 14 by flexible conductive leads 32, amplifies signals generated in the head 14 during a read operation to improve a signal-to-noise ratio of a read signal. In addition to the preamplifier 30, an arm electronics module (not shown in FIG. 1 but mounted proximate the preamplifier) may include circuits that switch the head function between read and write operations and write drivers that supply a write current to the head 14 during the write operation to store data on the platter 12. In one embodiment, the preamplifier is one element of the electronics module. The configuration and components of the arm electronics module and the preamplifier 30 may vary according to the system design as understood by persons familiar with such technology.
Data bits supplied to the disk drive 10 are stored on the platter 12 in sectors 40 of concentric tracks 42. Typically, a sector contains a fixed number of bytes (for example, 256 or 512). A plurality of sectors are commonly grouped into a duster.
FIG. 2 illustrates the platter 12 comprising a substrate 50 and a thin film 52 disposed thereover. The magnetic transducer or head 14 comprises a write head 14A for writing data bits to the disk 12 by altering magnetic domains of ferromagnetic material in the film 52, thereby creating magnetic transitions in the magnetic domains. A read head 14B reads the magnetic transitions to determine the stored data bit.
In other embodiments, the write head 14A and the read head 14B operate with other storage media (not shown) comprising a rigid magnetic disk, a flexible magnetic disk, magnetic tape and a magneto-optical disk.
The read head 14B is biased by a DC (direct current) voltage of about 0.3V supplied by the preamplifier 30 to read head terminals 54A and 54B via the conductive leads 32. The magnetic domains in the thin film 52 passing under the read head 14B alter a resistance of the magneto-resistive material, imposing an AC (alternating current) component in the DC bias voltage, wherein the AC component represents the read data bits. The AC component is supplied to the preamplifier 30 via the conductive leads 32. The AC component of the head output signal is relatively small (e.g., several millivolts) with respect to the DC bias voltage.
The susceptibility of certain integrated circuits to electrostatic discharge events is well known. An ESD event occurs when a charged object (e.g., a finger of a person handling the integrated circuit or a device for capturing and installing the integrated circuit into a printed circuit board) is disposed proximate an integrated circuit pin having a different potential than the charged object. If the potential difference is sufficient to breakdown insulating material separating the charged object and the pin (e.g., air) an electrostatic discharge is produced. Such discharges may generate a current exceeding one ampere during a period of less than 200 nanoseconds. The discharge current magnitude and waveform depend on the effective resistance, capacitance and inductance in the discharge path and the charge intensity present on the surfaces before the static discharge. The ESD event can destroy the integrated circuit by damaging substrate material or conductive interconnects in the integrated circuit. It is common practice to include ESD-protection components within the integrated circuit for directing the ESD current away from static-discharge sensitive components.
The disk drive read head 14B typically comprises either a magneto-resistive (MR) sensor or an inductive sensor. The MR sensor is more commonly used, especially in high-density disk drives, because the MR sensor generates a larger amplitude output signal than the inductive sensor, resulting in a higher signal-to-noise ratio in the read mode and a higher areal data storage density for the disk drive 10. However, when exposed to an ESD event or an electrical overstress (EOS) condition (i.e., an input voltage or current greater than expected under normal operating conditions), the MR sensor tends to be more susceptible to damage than its inductive counterpart due to the relatively small physical size of the MR sensing material. For example, a typical cross-section for an MR read sensor used for extremely high recording densities is about 100 Angstroms by 1.0 micrometer. An ESD event producing a discharge voltage of only a few hundred millivolts across such a small resistance is sufficient to produce currents capable of severely damaging or destroying the MR read head.
The read head 14B typically operates as a differential device, i.e., during a read operation the differential voltage across the signal terminals 54A and 54B represents the read data bits, with a voltage of a first polarity indicating a stored first logic level and a voltage of a second polarity indicating a stored second logic level. The read head 14B is thus extremely sensitive to ESD damage caused by a high differential voltage applied between the signal terminals 54A and 54B. A differential voltage as low as 0.5 volts can damage a state-of-the-art MR head due when ESD current flows through the head. A single relatively low magnitude ESD event or a series of relatively low magnitude events can degrade the magneto-resistive element, changing the resistance of the MR head and thus the head response during read operations, possibly causing data read errors. A relatively large ESD event can melt or evaporate the magneto-resistive element.
Given their high-ESD sensitivity, to prevent ESD/EOS damage, the MR sensor must be carefully handled during manufacture/assembly of the disk drive 10 and the read head 14B. Such ESD events are especially likely during manufacturing stages when the terminals 54A and 54B are exposed. For example, in a manufacturing process employing a rubber or plastic conveyor belt for transporting the head and associated components between manufacturing stations, ionized gas is dispersed over the conveyor belt to discharge electrostatic charges generated in the belt material.
During the disk drive assembly process the preamplifier 30 is connected to the head terminals 54A and 54B via the conductors 32A and 32B. To provide additional ESD protection for the read head 14B, it is advantageous for the preamplifier 30 to include one or more components to direct the ESD charge away from the MR read head 14B during the remainder of the assembly process. Since no power is supplied to the preamplifier 30 during the assembly operation, such components operate passively, i.e. they do not require the application of an external voltage. However, it is known that during disk drive operation parasitic capacitances produced by these passive components tend to degrade the read signal quality. This signal degradation becomes an increasingly troublesomeproblem as read data rates increase, it is therefore desired to employ ESD protection components that protect the read head 14B during assembly, without degrading preamplifier/head performance during operation.
One prior art technique for providing ESD protection for the differential signal terminals 54A and 54B (connected respectively to conductive leads 32A and 32B of the flexible conductive leads 32) is illustrated in FIG. 3. Diodes 70 and 72 are connected back-to-back (i.e., a cathode of a first diode is connected to anode of a second diode and an anode of the first diode is connected to a cathode of the second diode; also referred to as an anti-parallel configuration) to short or clamp the signal terminals 54A and 54B together in response to application of either a negative or a positive ESD voltage to either the terminal 54A or 54B. The diodes 70 and 72 provide adequate protection if the read head 14B can withstand a differential voltage greater than a diode turn-on voltage of about 0.8V, i.e., the voltage at which the diode becomes conductive and shorts the differential signal terminals 54A and 54B. Unfortunately, newer generation heads can fail at differential voltages below 0.8V. Although it may be possible to identify diodes fabricated from material providing a turn-on voltage below 0.8V, disadvantageously such a low turn-on voltage clips the differential head output signal if the diodes are driven into conduction during a read operation.
Another prior art technique as disclosed in U.S. Pat. No. 6,552,879 is illustrated in FIG. 4. A MOSFET (metal oxide semiconductor field effect transistor) 80, connected between the terminals 54A and 54B, is triggered to a conductive state, i.e., a low resistance path between a drain D and a source S, by a static charge sensing circuit 56 that triggers a gate G in response to the ESD voltage. The low resistance source-drain path effectively shorts the terminals 54A and 54B, preventing a voltage differential from developing therebetween.
The sensing circuit 56 adds cost and a space penalty to the disk drive 10 and requires a power source for operation. During disk drive assembly, power is not applied to the sensing circuit 56 and thus the circuit cannot provide ESD protection. To overcome the lack of a power source, in another embodiment the sensing circuit 56 is powered by the applied static pulse. But this embodiment requires a pulse amplitude larger than about 0.5V, in contravention of the requirement that the discharge protection circuit maintain the differential input voltage at less than about 0.5V.
Yet another prior art technique, illustrated in FIG. 5, comprises a fuse 84 connected across the terminals 54A and 54B. During disk drive assembly the fuse 84 shorts ESD current between the terminals 54A and 54B. After the head 14B is assembled by the disc drive manufacturer the fuse is opened. However, with the fuse short circuit precludes testing of the read head 14B when the head is in the form of an integrated circuit on a semiconductor wafer. Also, the fuse 84 does not provide a ground path for common mode charges induced across the terminals 54A and 54B.
According to another prior art technique, a depletion mode MOSFET 88 (see FIG. 6) is connected between the terminals 54A and 54B. It is known that a channel of the MOSFET 88 must be relatively large to minimize its “on” resistance and thereby reduce the ESD voltage (i.e., bleed the ESD charge) that is developed across the terminals 54A and 54B during an ESD event. If the “on” resistance is excessive then the voltage developed across the resistance can damage the read head 14B. However, as the MOSFET channel size increases, the parasitic capacitance introduced into the signal path between the read head 14B and the preamplifier 30 also increases. The parasitic capacitance reduces the operating bandwidth, a potential problem as disc drive heads are required to operate at higher data rates when reading data from the disk 12.